A prominent feature of Alzheimer's disease (AD) is the loss of neurons by apoptotic cell death. Apoptosis is characterized by plasma membrane blebbing, nuclear condensation, and DNA fragmentation and is initiated by the activation of caspases, a family of aspartate proteases. The initiation of apoptosis involves the sequential activation of pro-caspases to their active form by proteolysis. Two key members of this family are caspase-8, the most apical member of the caspases, and caspase-3 that is commonly referred to as the executioner member of this family. Because caspases are specific, cleaving after aspartic residues, this generates caspase cleavage products (CCPs) that are antigenically distinct and therefore, represent desirable targets for cleavage site-directed antibodies. Using this approach, we designed an antibody to CCPs of fodrin, a neuronal cytoskeleton protein, and showed widespread accumulation of these products in Alzheimer's disease. Thus, while no staining was observed in control cases, labeling of neurons was observed in the hippocampus and entorhinal cortex of all AD cases, which increased as a function of disease progression. This study along with others has demonstrated a prominent role for the activation of apoptotic mechanisms in neurons of the AD brain. Presently, there are two major pathways of apoptosis: the death receptor pathway in which caspase-8 plays a critical initiator role and the mitochondrial pathway involving oxidative stress and activation of caspase-9. Induction of cell death via the Fas/TNFR super family of death receptors is mediated by adapter proteins (e.g., Fas-associated death domain, FADD) and initiation caspases (e.g., caspase-8). In the present application we test the role of the death receptor pathway and caspase-8 in Alzheimer's disease. To examine the role of caspase-8 in Alzheimer's disease we will propose to 1) develop cleavage site-directed antibodies against the active fragments of caspase-8; 2) characterize these antibodies using model systems of apoptosis; 3) use this antibody together with the fodrin CCP antibody to determine the role of caspase-8 in mediating the activation of caspase-3 in neurons of the AD brain. Elucidation of the exact apoptotic pathway involved in the eventual activation of caspase-3 will lead to the identification of newer, more specific targets for pharmacological intervention that may be useful for the treatment of Alzheimer's disease.